This invention relates to a method and apparatus for non-destructively determining dimensional changes in a member, such as changes due to stresses or thermal expansion in structural members, or structural loads in buildings, bridges and the like, and even in machining, grinding, polishing or otherwise processing of a member.
An urgent need exists within many industries for a low cost precision method of installing structural members subject to stress particularly in high risk fracture control structures. The requirements for such a method are stringent and must account for a number of different types of structural members and installation conditions. In addition the method should be capable of both production line and field use. Further, the method should be capable of providing for repeat inspections with the information of previous inspections stored and retrieved for evaluating the effect of complex installation procedures and service load conditions.
A successful development of such a method requires the solution to a number of interrelated problems. For instance, the desired stress load to be controlled is the actual stress load existing in the member as opposed to other frictional or installation loads developed by the installation conditions, e.g., alignment effects, countersinks, surface roughness, etc. The interaction of these conditions on the load is extremely complex and must be analyzed to determine the effect on the actual stress load in the member. Additionally, the material characteristics of the structural system (fasteners, nut, washer, spacers) including physical properties (modulus, density, etc.) and geometric properties (size, tolerances, surface roughness, etc.) could significantly affect the accuracy and reliability of the load measuring method.
There are two nondestructive test methods that have been developed for determining stresses in metal members. These methods are X-ray diffraction and ultrasonics. X-ray diffraction techniques, which measure changes in the lattice spacing in known crystal systems require removal of material, and limitations are placed on the member geometry and the state of stress to be measured. Ultrasonic techniques measure the travel distance and the velocity of ultrasonic waves propagating in a member which is affected by the magnitude of the applied stress in the member.
An ultrasonic method is considered the most direct and accurate method for accurately measuring and controlling fastener load for the following reasons: the ultrasonic measurement is directly relatable to the tensile load in the fastener; the measurement is independent of the variable section modulus of the clamped structure and the "friction effects" caused by imperfect action between the fastener and nut; and very importantly, measurements can be made at any time in the life of the fastener without changing or affecting the load on the fastener. Some or all of these considerations are also applicable to other types of structural members.
Various systems using an ultrasonic method have been proposed for measuring tensile load in a fastener. Such a system disclosed in U.S. Pat. No. 3,759,090, titled Ultrasonic Extensometer, employs circuit means for analog measurement and display of a delay in receiving an echo pulse from the far end of a fastener. A complex system of multivibrators includes a vernier multivibrator employed to gate an oscillator on to produce periodic pulses at a fixed rate for an extended period from a time after the ultrasonic pulse has entered the fastener to a time after the echo pulse is received. The received echo pulse sets a timing multivibrator, and the next one of the vernier periodic pulses resets the timing multivibrator. The duration of the resulting output pulse from the timing multivibrator will decrease as the fastener is decreased in length (loosened) and increase as the fastener is increased in length (tightened). For ease of operation, the vernier multivibrator is set to produce adjacent periodic pulses spaced equal time intervals from an echo pulse. The output pulse of the timing multivibrator will then yield a pulse width modulated output proportional to the change in fastener length. Each output pulse is converted from pulse width to pulse height for display purposes.
The problem with such a system is that it relies upon the stability of multivibrators for timing, and therefore readjustment of the multivibrators each time it is used. Since it yields only change in length information when the system is used, it cannot provide length data that may be stored for comparison with tests made at a later date to determine if the fastener has undergone changes in stress. The system will provide data only on change in stress at the time the fastener is beng tightened or loosened. This method is also dependent on transit time through the coupling system and is subject to inaccuracies due to coupling variations during application of loads. Periodic maintenance tests on the stress of the fastener are not possible.
Another approach to measuring load stress (length) of a fastener is based upon the change in resonant frequency that a fastener undergoes due to any change in length. While this distinct approach utilized in the systems disclosed in U.S. Pat. Nos. 3,306,100 and 3,307,393 is capable of providing data on the absolute length of the fastener, as opposed to only change in length, the fastener is damped by the structural members being fastened, i.e., the true resonant frequency of the member is changed by coupling with fastened members. It is therefore believed that an ultrasonic pulsing technique will provide superior results as compared to any mechanical vibration technique.
Process monitoring of machining, grinding, polishing or other material processing of a member is desirable to determine precise amount and/or rate of material removal. The monitoring system should provide in-process real-time measurement and data for control of equipment performing the process. In addition, accurate certification of final material dimension(s) eliminates normal quality control inspection function. Such a monitoring system would provide more accurately processed material, thus producing savings from: eliminating the possibility of over processing material; more efficient processing to reduce total time; and saving inspection time by providing certified thickness data.
As steel framed buildings, bridges and the like are constructed, the loads in the structural members, such as "I" beams and other members, are equalized by adjustment of struts and braces. The compressive loads in the structural members as well as the tensile loads in the struts and braces should be monitored during installation, and in many cases during periodic inspection to assure proper construction, and could also be used for periodic checks to detect shifts or relaxation, and to study the effects of earthquakes and strong winds.